Saving energy and cost in the powertrain and reducing pollution from exhaust gas and noise emissions are current goals in vehicle development. In suitable driving situations, vehicles can be temporarily advanced in a rolling or coasting mode without being driven. When a motor vehicle is in rolling mode, the internal combustion engine can turn over with minimized consumption and low emissions while idling as the vehicle rolls with a disengaged drive train. If the internal combustion engine is turned off in such a driving situation, to further increase the resulting savings, the vehicle moves in a so-called coasting mode. These functions are already known.
Depending on the available drive train components and transmission design, the flow of power can be interrupted by shifting the transmission into neutral and/or by disengaging the start-off or lock-up clutch once rolling or coasting is possible. The rolling or coasting mode is ended by engaging the drive train or shifting into a gear.
It has proven to be problematic to identify suitable driving situations for activating the rolling or coasting function, simultaneously avoiding unnecessary interruptions in the flow of power, and terminating the function at the right time. If the function is terminated too early, available potential momentum can remain largely unused, and the resulting savings is lost. On the other hand, if the topography subsequently rises, especially for heavy commercial vehicles, the function can lead to insufficient traction and a slowdown of driving with downshifting to the extent of having to start off again. In addition, the driving comfort, wear of the related components in the drive train and driving safety need to be taken into account.
However, known methods for controlling the rolling or coasting function frequently incorrectly identify situations for interrupting and suspending the interruption of the flow of the force due to a somewhat unfiltered or insufficient analysis of the topographic conditions of the roadway, and/or an insufficient analysis of the current driving status of the motor vehicle, or the situations are too complex, and this can cause misinterpretations with activations and deactivations of the rolling or coasting function that, depending on the situation, are undesirable and possibly counterproductive for the driver, or with a rather inefficient exploitation of suitable rolling or coasting situations.
Furthermore, it is known that modern vehicles are increasingly being equipped with automatic vehicle speed control functions or vehicle speed functions and distance control functions for reasons of comfort, traffic, and vehicle safety. Such systems are known as so-called Tempomat/Bremsomat systems for example, and are available for passenger cars and commercial vehicles. They make it possible to maintain a desired speed dictated by the driver or when expanded as an ACC (adaptive cruise control) or ADR (automatic distance regulation), create a monitoring area in front of or around the vehicle with the assistance of sensors such as radar, infrared, video or ultrasound, whereby at least the distance from the preceding vehicle is adapted by means of a Bremsomat by automatically letting up on gas or braking.
In particular, known Tempomat/Bremsomat systems in commercial vehicles react by braking with an available auxiliary brake system when deceleration is required on a gradient and/or to adapt the distance from the preceding vehicle, such as an input-side or output-side hydrodynamic or electrodynamic retarder and/or the service break, where the retarder is preferably used. If acceleration is needed to maintain or reach a specified Tempomat speed, these systems request engine torque.
A method is known from DE 102 21 701 A1 to control a rolling or coasting function for a motor vehicle with an automatic transmission. With the known method, a quantity representing the vehicle speed, brake actuation and/or actuation of a fuel supply measuring element and an operating state of the internal combustion engine are detected. A clutch device is disengaged, thereby interrupting the flow of force in the drive train when neither the gas pedal, nor the fuel supply measuring element, nor the brake pedal are actuated while the internal combustion engine is running and a specific vehicle speed is exceeded. After the clutch is disengaged, the transmission is shifted into neutral and the vehicle is therefore in a rolling mode.
When the gas pedal or the brake pedal is actuated, or variables having like effect are changed, the clutch is re-engaged, thereby terminating the rolling mode. Before the clutch is engaged, the rotational speeds of the internal combustion engine and a transmission input shaft are first synchronized to return from rolling mode to regular drive as smoothly as possible. The disengagement of the clutch and hence rolling mode are prevented when a probable need for braking is assumed. This can, for example, be the case while traveling downhill, when the gas pedal is suddenly released, or when a sporty driving program is selected.
A method is known from DE 10 2007 001 936 A1 to control a shifting clutch of an automatic transmission of a motor vehicle in which hill detection and neutral idle controls are continuously evaluated and checked. The method is executed in order to save fuel in a neutral transmission position during idling states of the internal combustion engine, while stopping at a signal, or in stop-and-go operation. At the same time, the vehicle is prevented from rolling backward on a slope when there is not enough brake pressure to hold the vehicle when the transmission shifts into neutral.
Certain initial conditions have to be satisfied for such a neutral idle control, that is, situation-controlled neutral shifting, which includes the evaluation of a hill detection flag and a hill counter. A hill detection flag can be set when there is a corresponding evaluation using hill detection, or a hill counter can be incremented. This is the case when the transmission is in a forward gear, the rotational speed of the transmission output shaft is greater than a specified output shaft rotational speed limit, a throttle position of the internal combustion engine is less than a specified throttle position limit, a brake status indicates that the vehicle brake is actuated, and the transmission temperature is within a permissible range.
Rollback-free neutral idle shifting is possible when the current counter reading is less than a specified counter limit, or when the brake pressure is greater than a precalculated brake pressure limit when the counter reading is above this counter limit, or if the brake pressure is not available and the counter reading is greater than the counter limit when a hill detection flag is set.
DE 10 2004 017 115 A1 makes known a method for the automatic driving speed control or driving speed and distance control for a vehicle comprising an automatic or automated transmission, in which the driving speed can vary within the range of a preselected target speed. In that particular case, an acceleration phase is followed by a roll-out phase in which the flow of power in the drive train can be interrupted to save fuel. Topographical data, data from a monitoring device pertaining to the immediate vehicle surroundings, and/or certain vehicle parameters can be taken into account via a navigation device in the sequence of acceleration and roll-out phases. The acceleration phases can be supported by an electric motor which is present in addition to an internal combustion engine.
WO 2003/037672 makes known a multi-speed transmission, in particular for heavy commercial vehicles, which is shifted into neutral, i.e. into a rolling mode, when a low-consumption driving state is detected. A low-consumption driving state is detected and the neutral position is attained via shifting when a target speed is specified and the driving speed would decrease without the rolling mode. This is assumed, in particular, when neither the gas pedal nor the brake pedal is pressed, and equivalent variables do not change. A supplemental brake system which can be requested in rolling mode is provided for safety reasons. The gear that is currently engaged is also taken into consideration.
In addition, a method is known from WO 2009/037167 A1 for controlling an automated or automatic multi-step transmission of a motor vehicle, especially a heavy commercial vehicle, in which a higher gear is shifted into before the gradient taper or before starting traction mode, that is, a thrust upshift is executed, to reduce fuel consumption while driving down a slope and then leaving the slope instead of only upshifting after leveling out or transitioning from thrust mode to traction mode as usual.
The current roadway gradient is determined in a specified interval and a function is activated when driving downhill to identify the gradient taper to automatically upshift as early as possible. The rotational speed of the drive motor and possibly of an active constant braking device is thereby reduced which could cause the motor vehicle to accelerate due to the lower motor drag torque and possibly lower retarder braking torque, thereby causing the traction mode to start comparatively later. The higher gear is selected so that the motor vehicle only accelerates slightly, and the driver can control the situation safely at all times.
The gradient taper is identified when the just traveled gradient decreases steadily over a specific number of intervals and falls below a threshold. In addition, various criteria can be provided such as the actuation of the service brake, excessive vehicle acceleration, an excessively steep gradient, a particularly heavy load, the operation of an auxiliary brake at its load limit or falling below a target speed or falling below a safe distance regulated by an active Tempomat/Bremsomat that, for reasons of safety, lead to the blocking of thrust upshifting.
The known methods have various options for reducing fuel consumption with the assistance of driving with flywheel momentum. However, a person skilled in the art would not find any suggestions therein of terminating a rolling or coasting function by considering the effect of a vehicle's speed regulating device.